Faculty Profiles - OZAKI Nobuhiko

Published Papers - OZAKI Nobuhiko

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Numerical validation of low-coherence interferometers using low-group-velocity and low-dispersion photonic crystal waveguides for ultra-compact OCT applications

Nanaho Oda, Hisaya Oda, Naoki Ikeda, and Nobuhiko Ozaki （Part： Last author,　Corresponding author )

Japanese Journal of Applied Physics ( IOP publishing ) 64 ( 4 ) 04SP58 1-6 2025.04 [Refereed]

In this study, an ultra-compact optical coherence tomography (OCT) system based on integrated photonic crystal waveguides (PhC-WGs) was proposed and validated numerically. A low-velocity and low-dispersion (LVLD) PhC-WG is employed in the reference arm, enabling significant reduction in its length because of the high group index achieved with a broadband light source. Optical delays in the reference arm were found to increase with the designed group index of the LVLD-PhC-WG, confirming the low-coherence interference. Additionally, similar findings were observed in a PhC-WG-based OCT (PhC-OCT) model integrating a directional coupler and the LVLD-PhC-WG. The PhC-OCT with a considerably small footprint, e.g. 20 × 100 μm2, for the interferometer including the reference arm indicated interferometric signals with an axial resolution of approximately 40 μm. These results demonstrate the feasibility of an ultra-compact OCT system using PhC-WGs.

Monolithic vertical cavities via selective-area growth using a rotational metal mask and InAs quantum dots for near-infrared multiple-wavelength surface-emitting light source applications

Yuuki C. Hodson, Tatsuki Yokota, Eiichiro Watanabe, Naoki Ikeda, and Nobuhiko Ozaki （Part： Last author,　Corresponding author )

Applied Physics Express ( IOP Publishing ) 18 ( 3 ) 035501 1-5 2025.03 [Refereed]

We herein report a method for fabricating near-infrared multiple-wavelength surface-emitting light sources. After growing a GaAs layer containing stacked InAs quantum dots (QDs) on a GaAs/AlAs distributed Bragg reflector by molecular beam epitaxy, additional GaAs layers were grown via selective-area growth using a rotational metal-mask. The GaAs thickness variation generated different vertical cavity (VC) modes that resonated with different emission wavelengths within the broadband emission from the InAs QDs. Photoluminescence measurements confirmed monolithic VCs formation in the selected areas. This method offers a simple fabrication process and extends the emission wavelength range of near-infrared multiple-wavelength surface-emitting light sources.

In segregation influence on properties of InAs quantum dots in dots-in-a-well

Koki Okuno, Naoki Okada, Kosuke Iwaide, and Nobuhiko Ozaki （Part： Last author,　Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 63 ( 5 ) 055507 1-5 2024.05 [Refereed]

We investigated the growth of InAs quantum dots (QDs) in a dots-in-a-well (DWELL) from the perspective of the influence of In segregation from the InGaAs layers in the DWELL. Reflection high-energy electron diffraction (RHEED) measurements during the growth of the lower InGaAs layer indicated that In segregation increased with the In composition of the InGaAs layer. The estimated In segregation values were consistent with the decreases in the critical thickness for the QDs growth and the total volume variations of the grown QDs. These results illustrate that the segregated In from the lower InGaAs layer contributes to the QD growth in the DWELL, and their density increases. Furthermore, RHEED measurements during the growth of the upper InGaAs layer indicated the suppression of the deformation of embedded QDs , which could partially contribute to the longer emission wavelength of the QDs in the DWELL.

Proposal and numerical verification of an ultrasmall terahertz source using integrated photonic crystal waveguides for highly efficient differential frequency generation

Yota Koyama, Hisaya Oda, Naoki Ikeda, Yoshimasa Sugimoto, Nobuhiko Ozaki （Part： Last author,　Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 62 ( SG ) SG1033 1-5 2023.03 [Refereed]

We propose and numerically investigate integrated photonic crystal waveguides (PhC-WGs) formed in a semiconductor slab to realize an ultrasmall and highly efficient terahertz (THz) wave source. The structure consists of a straight PhC-WG with low-group-velocity and low-dispersion (LVLD) for efficient difference frequency generation (DFG) connected to two PhC-WGs to introduce two fundamental lights into the LVLD PhC-WG. The fundamental light propagating through each PhC-WG designed to enhance their electric fields by the slow-light effect is efficiently coupled to the LVLD PhC-WG owing to the reduced refractive index differences at the boundaries of the heterostructures. The DFG from the two fundamental lights was numerically simulated, and a temporal intensity oscillation corresponding to the difference in frequency was clearly observed. By comparing the DFG intensities of the integrated structures with an LVLD PhC-WG and a strip WG, the estimated DFG intensity from the LVLD PhC-WG was more than 100 times higher than that from the strip WG. These results indicate the effectiveness of the proposed heterostructure in the application of a highly efficient THz source with an ultrasmall footprint compared with conventional materials.

Broadband optical wavelength conversion through four-wave mixing in W3-type AlGaAs photonic crystal waveguides

Hisaya Oda, Youhei Hosokawa, Kazuki Hayashi, Nobuhiko Ozaki, Naoki Ikeda, Yoshimasa Sugimoto

Applied Physics Express ( IOP Publishing ) 16 ( 2 ) 022004 1-4 2023.02 [Refereed]

We realized a wide-band wavelength conversion method through four-wave mixing in W3-type AlGaAs photonic crystal waveguides. AlGaAs exhibits a large third-order nonlinearity. Furthermore, because of its large bandgap, two-photon absorption can be avoided in the 1550-nm range. A four-wave mixing efficiency of −9.03 dB was obtained for a pump peak power of 7 W. Furthermore, by utilizing the two even guided bands of the W3-type photonic crystal waveguide, a conversion bandwidth greater than 38 nm was achieved with a conversion efficiency of −22 dB.

In-situ estimation of emission wavelength of embedded InAs QDs using RHEED intensity measurements

Nobuhiko Ozaki, Daigo Ikuno （Part： Lead author,　Corresponding author )

Journal of Crystal Growth ( Elsevier BV ) 588 126657 1-4 2022.06 [Refereed]

We propose a method for estimating the emission wavelength of self-assembled InAs quantum dots (QDs) embedded in a capping layer using reflection high-energy electron diffraction (RHEED) intensity measurements. As QDs are capped with a Ga(In)As layer, the RHEED intensity of QDs decays, and the decay degree depends on the growth conditions of capping. The RHEED intensity decay was analyzed, and a relationship between the decay degree and photoluminescence peak energy of embedded QDs was determined. This relationship can be used to predict the emission wavelength of embedded QDs during capping, which is particularly useful for QD-based optical device fabrication.

Near-infrared dual-wavelength surface-emitting light source using InAs quantum dots resonant with vertical cavity modes

Jin Oshima, Nobuhiko Ozaki, Hisaya Oda, Eiichiro Watanabe, Hirotaka Ohsato, Naoki Ikeda, Yoshimasa Sugimoto, Richard A. Hogg （Part： Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 61 ( SD ) SD1003 1-5 2022.03 [Refereed]

We developed a compact dual-wavelength surface-emitting light source using InAs quantum dots (QDs) embedded in a vertical cavity (VC). The VC was designed to possess two optical cavity modes that resonate with the discrete emission lines of the QDs. The fabricated light source exhibited significant enhancements in the vertical light emission corresponding to the VC modes. In addition, the light source demonstrated selectivity to the enhanced emission wavelengths with changes in temperature. Compared to conventional dual-wavelength vertical external cavity surface-emitting lasers, these QD-based dual-wavelength emission devices allow for the realization of simple structures because the InAs QDs act as dual-light-emitting materials. These results can be applied to simple dual-wavelength surface-emitting light sources.

Emission wavelength control of InAs/GaAs quantum dots using an As2 source for near-infrared broadband light source applications

Nobuhiko Ozaki, Yuma Hayashi, Shunsuke Ohkouchi, Hirotaka Ohsato, Eiichiro Watanabe, Naoki Ikeda, Yoshimasa Sugimoto, Richard A. Hogg （Part： Lead author,　Corresponding author )

Applied Physics Express ( IOP Publishing ) 14 ( 5 ) 055501 1-6 2021.04 [Refereed]

Herein, we report an emission wavelength control technique for self-assembled InAs quantum dots (QDs) grown via molecular beam epitaxy using an As2 source (As2-QDs). The As2-QDs exhibited photoluminescence with a shorter center wavelength and larger bandwidth than those of the QDs grown using an As4 source. In addition, the emission center wavelength could be controlled by adjusting the time between the growth and capping of the As2-QDs. We utilized the multilayer stack of emission-wavelength-controlled As2-QDs to fabricate an electrically-driven light source and demonstrated its broadband (approximately 130 nm) emission in the 1–1.3 μm wavelength range.

1.1 μm waveband tunable laser using emission-wavelength-controlled InAs quantum dots for swept-source optical coherence tomography applications

Toshiya Tsuji, Nobuhiko Ozaki, Sho Yamauchi, Katsuya Onoue, Eiichiro Watanabe, Hirotaka Ohsato, Naoki Ikeda, Yoshimasa Sugimoto, David T. D. Childs, and Richard A. Hogg （Part： Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 60 ( SB ) SBBE02 1-6 2021.02 [Refereed]

In this study, an optical gain chip using emission-wavelength-controlled self-assembled InAs quantum dots (QDs) was developed for swept-source optical coherence tomography (SS-OCT) applications. The optical characterizations indicated that the QDs emission wavelength and optical gain spectra were controlled in the 1.1μm waveband by optimizing the QDs growth conditions. This waveband is useful for obtaining a large imaging depth of OCT because of an optimal balance between absorption and scattering in biological samples. In addition, continuous tunable lasing in the waveband was achieved by introducing the QD-based gain chip into a grating-coupled external cavity. This tunable laser was introduced into an SS-OCT setup, and the point spread function (PSF) was evaluated. The PSF position was observed to vary according to the optical path length differences. These results demonstrate the feasibility of the application of emission-wavelength-controlled QDs for SS-OCT.

Bandwidth enhancement in an InGaN/GaN three-section superluminescent diode for optical coherence tomography

Graham R. Goldberg, Dae-Hyun Kim, Richard J. E. Taylor, David T. D. Childs, Pavlo Ivanov, Nobuhiko Ozaki, Kenneth L. Kennedy, Kristian M. Groom, Yukihiro Harada, and Richard A. Hogg

Applied Physics Letters ( AIP Publishing ) 117 ( 6 ) 061106 1-5 2020.08 [Refereed]

In this paper, the optoelectronic properties of InGaN-based blue (430 nm) superluminescent light-emitting diodes with a multi-section, three contact design are reported. The bias conditions of the rear absorber section and gain sections are explored in terms of enhancing and maximizing spectral bandwidth. We demonstrate that broader emission can be obtained with a short circuit, rather than an open circuit absorber section, and with two active regions at different current densities as opposed to a single active contact. Under optimal drive conditions, a −3 dB linewidth of 20 nm is obtained at 430 nm. Analysis of the device emission spectrum indicates that an axial resolution of ∼3.4 μm may be obtained in an optical coherence tomography system.

Numerical investigation of highly efficient and tunable terahertz-wave generation using a low-group-velocity and low-dispersion two-dimensional GaAs photonic crystal waveguide

Teruyuki Nakahama, Nobuhiko Ozaki, Hisaya Oda, Naoki Ikeda, and Yoshimasa Sugimoto （Part： Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 59 ( 9 ) 090903 1-7 2020.08 [Refereed]

Noninvasive High-axial-resolution Profile Imaging Using Near Infrared Broadband Light Source Based on Self-assembled Quantum Dots

Nobuhiko Ozaki （Part： Lead author,　Corresponding author )

Journal of the Imaging Society of Japan 58 ( 6 ) 617 - 625 2019.12 [Refereed] [Invited]

Development of a broadband superluminescent diode based on self-assembled InAs quantum dots and demonstration of high-axial-resolution optical coherence tomography imaging

Nobuhiko Ozaki, Sho Yamauchi, Yuma Hayashi, Eiichiro Watanabe, Hirotaka Ohsato, Naoki Ikeda, Yoshimasa Sugimoto, Kenji Furuki, Yoichi Oikawa, Kunio Miyaji, David T D Childs, and Richard A Hogg （Part： Lead author,　Corresponding author )

Journal of Physics D: Applied Physics ( IOP Publishing ) 52 ( 22 ) 225105 1-9 2019.05 [Refereed]

Tunable external cavity laser diode based on wavelength controlled self-assembled InAs quantum dots for swept-source optical coherence tomography applications at 1100 nm wavelength band

Nobuhiko Ozaki, David T. D. Childs, Aleksandr Boldin, Daigo Ikuno, Katsuya Onoue, Hirotaka Ohsato, Eiichiro Watanabe, Naoki Ikeda, Yoshimasa Sugimoto, Richard A. Hogg （Part： Lead author,　Corresponding author )

SPIE Proceedings ( SPIE ) 10939 1093911 1-6 2019.03 [Refereed]

Preparation of Ag2Se QDs with excellent aqueous dispersion stability by organic coating with aqueous ATRP

Yoshio Nakahara, Yuki Kunitsu, Nobuhiko Ozaki, Mutsuo Tanaka, Setsuko Yajima

Polymer Bulletin ( Springer Science and Business Media LLC ) 76 ( 9 ) 4753 - 4768 2018.12 [Refereed]

Non-destructive and non-contact measurement of semiconductor optical waveguide using optical coherence tomography with a visible broadband light source

Kazumasa Ishida, Nobuhiko Ozaki, Hirotaka Ohsato, Eiichiro Watanabe, Naoki Ikeda, Yoshimasa Sugimoto （Part： Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 57 ( 8S2 ) 08PE03 1-5 2018.08 [Refereed]

Reflection Spectra from SiC Substrate with Circular-Slot Antennas and Possible Interaction between Surface-Plasmon Polaritons and Surface-Phonon Polaritons

N. Umemori, K. Kasahara, T. Yaji, N. Ozaki, N. Ikeda, Y. Sugimoto

2018 12th International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials) ( IEEE ) 204 - 206 2018.08

Non-destructive inspection of semiconductor optical waveguide using optical coherence tomography with visible broadband light source

Kazumasa Ishida, Nobuhiko Ozaki, Naoki Ikeda, Yoshimasa Sugimoto

22nd Microoptics Conference, MOC 2017 ( Institute of Electrical and Electronics Engineers Inc. ) 2017- 242 - 243 2018.01 [Refereed]

Growth of quantum three-dimensional structure of InGaAs emitting at ~1 µm applicable for a broadband near-infrared light source

Nobuhiko Ozaki, Shingo Kanehira, Yuma Hayashi, Shunsuke Ohkouchi, Naoki Ikeda, Yoshimasa Sugimoto, Richard A. Hogg （Part： Lead author,　Corresponding author )

Journal of Crystal Growth ( Elsevier BV ) 477 230 - 234 2017.11 [Refereed]

Gallium Nitride Superluminescent Light Emitting Diodes for Optical Coherence Tomography Applications

Graham R. Goldberg, Aleksandr Boldin, Sophia M. L. Andersson, Pavlo Ivanov, Nobuhiko Ozaki, Richard J. E. Taylor, David T. D. Childs, Kristian M. Groom, Kenneth L. Kennedy, Richard A. Hogg

IEEE Journal of Selected Topics in Quantum Electronics ( Institute of Electrical and Electronics Engineers (IEEE) ) 23 ( 6 ) 2000511 1-11 2017.07 [Refereed]

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